Cold cathode light source



OC- 4, 1960 L. THoRlNGToN v 2,955,221

com cATHoDE LIGHT souRcE Filed April 2, 1959 Tij. l

Il IE-LTE.

' INVENTOR. wos f/o/A/Gm/v "fmLfm va magnesium oxide cold cathode.

COLD CATHODE LIGHT SOURCE Luke Thorington, Berkeley. Heights, NJ., assignor to Duro-Test Corporation, North Bergen, NJ., a corporation of New York Filed Apr. 2, 1959, Ser. No. 803,756

Claims. (Cl. 313-108) This invention comprises a light source of the type using a'cold cathode and employing cathodo-luminescence, as the source of light.

An object of this invention is to provide in a simplified form a cathodo-luminescent light source employing a cold cathode.'

Another object of the invention is to provide a novel combination for initiating operation of such light sources.

A more specific object of the invention is to employ the phenomenon of electro-luminescence for initiating the operation of cathodo-luminescent light sources.

Still another object of the invention is to pro-vide a light source of this type constructed to provide high intensity illumination.

Other and more detailed objects of the invention will -be apparent from the following description of the em- -bodiment illustrated in the attached drawings for" explanatory purposes.

In the drawings,

Figure 1 is a typical, longitudinal, vertical, cross-sectional view through one form of light source in accordance with this invention in which the elements are diagrammatically illustrated;

Figure 2 is a similar diagrammatic illustration in cross-section of another form of such light source designed to provide high intensity illumination;

Figure 3 is a similar View of a third form of the invention in which in principle'there is a reversal of the relationship of the parts of the light source of Fig. 2; and

Figure 4 is a typical cross-sectional view of a fourth form of the invention.

The light source of this invention employs a cold cathode of the type which can be generally referred to as The electron emission of such cathode is employed to excite cathodo-lumines- 'cent phosphors to produce visible and useful light.

Electron tubes employing such a cathode are known but have heretofore in their practical forms required auxiliary starting apparatus, usually in the form of a tungsten filament, and it is the purpose of this invention to eliminate this starting element.

In accordance with this object it is proposed to initiate the operation of the cold cathode by the use of electroluminescent materials, which in a practical form of the lamp are incorporated in the cathodo-luminescent layer. The electro-luminescent and cathodo-luminescent materials could take the form of a coating in which these materials are present as a mechanical mixture. In another form the fluorescent layer may consist of a single phosphor which exhibits both electro-luminescent and cathodo-luminescent properties.

To better understand these objectives the several forms of lamps illustrated in the drawings will be described in detail. Y

As is usual with light sources of this type, there is provided a light transmitting envelope 10, usually of glass, having any physical shape and dimensions suited to the particular use proposed for the light source. The

States Patent() s 2,955,221 EQ@ Patented Oct- 4, 1960 envelope 10 is evacuated so as to be `as free as practicable of air and particularly oxygen. As illustrated in Fig. l, the cathode 12 comprises a body of `suitable metal which may be of nickel, a nickel-plated metal such as iron, or may even be a sheet of glass or other ceramic having a nickel coating. Assuming that the body 12 is a rectangular-plate, it is provided on its lower face, as shown at 14, with a magnesium oxide film. In the case where the body 12 is non-conducting it would first be provided with a thin coating of nickel which would Ibe overlaid by the magnesium oxide coating 14.

` There are a number of ways to form the magnesium oxide film or coating 14 which are available to those skilled in the art, and they will not, therefore, be detailed herein. It is understood, of course, that the film is very thin, having a thickness of the order of 10-3 to 104l cm. These dimensions are not given here in a critical sense, any cold cathode of this type is suited to the purposes of this invention.

As is well known, such cold cathodes are rendered active by providing a collector electrode facing the magnesium oxide film and maintained at a positive potential of from 200 to 300 volts with respect to the body 12 or the conducting coating underlying the film in the case of a non-conducting body. The collector electrode under potential creates a high intensity electric field across the thin magnesium oxide layer and when the magnesium oxide film is charged to the collector potential it starts and continues to emit electrons. The oxide film is charged by light, by the discharge of a Tesla coil or by a radioactive source. By varying the collector voltage the electron emission can be controlled from a few micro amperes to several tens of milliamperes, changing exponentially with the collector voltage. As is well known a `blue fluorescence appears on the film during emission and its variation in uniformity of brightness is a measure of the uniformity of emission over the surface area of Vthe film. Some electron tubes use in conjunction with the collector electrode an incandescent lilament to aid in the starting of the electron emission. Such starting filament is eliminated in accordance with an object of this invention.

The lamp includes a second body 16, in this case light transmitting, and hence preferably of glass. The

-glass is of the so-called conducting type, that is it has a thin transparent conducting tin oxide -coating 18 over the face thereof, which faces the lm 14, as clearly shown in Fig. 1. The conductive coating 18 is transparent and is Very thin.

Deposited on the exposed face of the layer 118 is a phosphor coating illustrated as consisting of a mixture of a oathodo-luminescent phosphor 2f) and an electroluminescent phosphor 22. The coating consists of a ho- .rnogeneous ymixture of those phosphors in powder form ing between the two electrodes so that the phosphor coating 20-22 is spaced close to the magnesium oxide film 114.

When la potential of from 200 to 300 volts is applied across the connections 24 and 26, the electro-luminescent particles 22 of the coating arecaused to luminesce, thereby sensitizing the magnesium oxide coating 14 which then starts emitting electrons. The sequence of operation is as follows: the Vpotential of 200 to 300 volts is applied between the connectors 24 and 26; the electro-luminescent particles emit light; the light causes the magnesium oxide cathode to start emitting electrons under he influence of the collector potential; `and these electrons are drawn to the anode `(the collecter) and bombard Ythecathodoluminescent layer resulting in the efcient generation of light.

e Specifically these electrons Vflow from the film 14 to the coating Ztl-22, Vbombarding the cathode-luminescent content 2Q of the phosphor layer causing it to luminesce. The luminescence of the electro-luminescent material is very low as such materials are known to be ineflcient light sources. On the other hand, the cathode-luminescent content of the Vphosphor `layer is an etlicient light generator under electron excitation. The resulting luminescence is transmitted through'the transparent body 16, and of course, through the housing for use. This type 4of' lamp, as will be apparent, is suitable for panel lighting, for example, where the bodies 12 and 16 can be in the form of six rectangular plates of any suitable and practicable dimension.

The housing l@ (as well as the housings 11d and 2l@ to be referred to later) diagramatically illustrates one way in which the active elements of the lamp are confined within a vacuum. However, Iin a practical lamp of this type a seal between the two elements 12 and 16 can be formed around the edges thereof and vacuum created between them to produce the completed lamp,'eliminat ing in such case the larger housing 16, for example.

Such a seal could be formed between Vthe-elements by using a low melting solder glass, of which different types are commercially available.V The coatings 14, 18, 2t) and 22 would, of course, terminate short of the edgesV of the plates 12 and lo, so that a seal directly to the plates could be made. The space between the plates when sealed would of course be evacuated by the required operating conditions, which evacuation can be etected in accordance with well known practices. 1t is emphasized that the active surfaces 14, 2t! and 22 will be much closer together inV an actual lamp. Relatively wide spacing is shown inthe drawing to facilitate illustration. s

The art is familiar with phosphors suitable to this purpose which include phosphors such Yas ZnS:Cu, ZnS:Ag, ZnCdS:Cu, ZnO, zinc silicate Mn. Some phosphors are both electro andY cathode-luminescent, and of 'those Ylisted above, yzinc sulphide activated with copper is such a phosphor. When this phosphor isursed it is the sole constituent ot the coating of the conductiveV layer 1,8. As those skilled in theV art will appreciate, there are other phosphore having either or both of these characteristics which are suited to the purposes of this invention.

The light source Fig. 2 is designed to be one of high intensity. As before, itY comprises Yan evacuated envelope 11i) of suitable coniigur-ation and dimensions. Within the envelope is a metal cylinder l112 having the magnesium oxide hlm 114 on its interior surface, as illustrated. As in the previous case, the cylinder 112 can be of nickel, a nickel plated metal, or of a ceramic having a nickel coating underlying the magnesium oxide film lll/4.Y

Positioned within the cylinder 112 is a rod V116 of accurately formed glass or quartz, having a transparent conductive coating llirde'posited on the surface thereof, like the coating 1S of the previous form. 1n cases where heavyV currents are encountered, a silver strip, not shown, may run the length of the rod and'underlie the coating 118. On the surface of the coating 118 is the phosphor coating comprising a mixture of cathode-luminescence and electro-luminescent materials 121)V and 122, as before.

The thin coating of phosphor maybe enclosed within a highly renective aluminum coating ZS'Which extends around one endV of the rod, as shown in Fig. 2. The. opposits end Si? of this rod is ground or polished. Also as shown i-n Fig. 2, the spacing between the interior iilmed Y surface ofthe cylinder i12-and the Vexterior coated sur-` face of the rod 116 is very small. Y As in the Ycase of the lamp of Fig. l, a suitable source of the proper voltage is connected between the leads and 124 to create a field, as in the previous case. -Inthe case of this lamp, Where the aluminum coating 2S is used, the potential applied to the leads V12d and 126 must be considerably higher than in the case where no reector is used, in order that the electrons may penetrate it. This has been indicated by the symbol KV and represents voltages of several kilovolts or higher, in order to get proper operation. In such a lamp an additional collector must be imposed between the cathode and anode and a potential of the order of 20S to 300 volts Iapplied across the leads 126 and 132 in order to prevent cathode breakdown.

Under the induence of this field the electro-luminescent material 122 causes electron emission from the magnesium oxide iilm M4, which emission in turn bombards the cathode-luminescent material 120 to render it luminescent. This light is reected by the reflecting coating 2S into the rod M6 through which it is transmitted to the point of emission at the end 30 of the rod. The light intensity emitted is of a very high order, depending, of course, upon the geometry of the elements of the lamp. indeed, arlamp of this form emits light of the order of a carbon arc or greater. The reliective coating 28 being thin, does not seriously interfere with the movement of electrons between the charged surfaces.

The lamp of Fig. 3 is quite similar to that of Fig. 2, but structurally involves a reversal of the parts, so tospeak, so that the light is emitted radially through theside walls of the envelope rather than through the end. In this case the lamp includes the evacuated envelope 210 as before. The cold cathode is in the form of a rod or tube 212, having the magnesium oxide iilm 214 on the outer surface thereof. This rod or tube lies concentrically of a cylinder 216 of glass having the transparent conductive coating 213 on the inner surface thereof. Deposited on this surface is the phosphor mixture comprising the'cathodoluminescent and the electro-luminescent constituent 220 and 22. As in the previous case a high voltage source is connected to the lamp elements through the leads 224 and 226. Y

ln this' case the luminescence of the cathode-luminescent material is transmitted through the transparent conducting coating 213 and the glass cylinder 216 to issue radially thereof through the surrounding Wall of the envelope 210. Of course, in this form of the `lamp as in the other two forms the magnesium oxide film 214 is positioned closely adjacent to the layer 220--222 so that the electric field from the charge electrodes will have suicient intensity to activate the electro-luminescent material, and thus initiate operation of the lamp.

Those skilled in this art'will appreciate that the prinoiples of this invention are applicable to physical lamp forms of a wide'variety of sizes and shapes, and the details o-t' the Vinvention are subject to considerable variation within the knowledge'of the art. VIt is preferred, therefore, that 4the subject matter of this invention be `limited by the scope of the Vappended claims rather than by the examples given for explanatory purposes. In this connection it can properly be saidthat the phosphor coated element of each lamp acts as an anode with respect to the' magnesium oxide coated'cold cathode.

'As an indication of these possibilities, reference is made Vto Fig. 4, wherein a form of light source is illustrated which is basically that of Fig. 1, but wherein a separate envelope Vhas been omitted. Since' the device of Fig. 4 is quite similar to that of Fig. lin so far as the same parts are used, the same reference characters have been applied. In this form the plate 12 has been indicated as being made of'a light transmitting material such as glass, but as pointed out above with regard to'Fig. `1, it can also be made of metal. In this arrangement the maihrdifference is'in the fact that, as suggested above, a seal can be provided around the` edges of the -plates 12 .and 13 'EQ Aform the chamber 30 within which the active elements are mounted. This chamber as suggested, can be evacuated or otherwise prepared in accordance with knowledge in the art, to provide the proper atmosphere for these active elements. In order to provide the chamber 30, a seal which has been diagrammaticatlly illustrated at 28 in the Ifigure, is provided around a'l'l of the edges of the plates to join them together in a manner to render the joint hermetically tight. Here again, those skilled in the art will see obvious ways of sealing the plates at the edges for this purpose, and so the illustration in Fig. 4 -is diagrammatic.

Fig. 4 also illustrates the point brought out above that in the actual llight source the conductive layer 14 and the cathode-luminescent 'layer 20v-22 will be quite close t0- gether for reasons understood in this alt.

What is claimed is:

1. A luminescent light source comprising a magnesium oxide type of cold cathode emitter, a light transmitting unit having a luminescent coating facing and positioned close to said cathode, an evacuated light transmitting envelope enclosing said cathode and anode, and means including sa-id coating for rendering said cathode electron emissive.

2. In -the combination of claim 1, said last means including a high voltage source connected across said cathode and anode.

3. In the combination of claim l, said coating having properties of electroand cathode-luminescence.

4. In the combination of claim 1, said luminescent coating comprising ZnSzCu.

5. In the combination of claim 1, said coating comprising a mixture of electro-luminescent and cathodoluminescent phosphors.

6. In the combination of claim 1, said cathode and anode comprising parallel plates.

7. In the combination of claim 1, said cathode and anode comprising parallel plates, sai-d anode being light transmitting.

8. In the combination of claim 1, said cathode being of cylindrical form and said anode comprising a light transmitting rod lying concentrically therewithin.

9. In the combination of claim l, said cathode and anode comprising concentrical'ly arranged cylindrical bodies.

l0. In the combination of claim 1, said anode comprising a light transmitting sleeve having said luminescent coating on its surface and said cathode being arranged concentrical-ly thereof.

11. In the combination of claim 1, said anode comprising a light transmitting sleeve having said luminescent coating on its inner surface, and said cathode comprising a rod concentricaflly thereof.

12. In the combination of claim 10, a collector grid interposed between said cathode and anode maintained at several hundred volts positive with respect to said cathode.

13. In the combination of claim 11, a collector grid interposed between said cathode and anode maintained at several hundred volts positive with respect to said cathode.

14. A luminescent Ilight source comprising a member providing amagnesium oxide type of cold cathode emitter, another member having a luminescent layer facing and closely spaced from said cathode, means including said layer for causing said cathode to become emissive, and means uniting said members to form a chamber to be evacuated between them.

15. In the combination of claim 14, said members being of light transmitting material.

References Cited in the le of this patent UNITED STATES PATENTS' 2,177,735 Meyer Oct. 31, 1939 2,222,668 Knoll 'Nov. 26, 1940 2,566,349 Mager Sept. 4, '1 2,783,407 Vierkotter Feb. 26, 1957 2,789,240 Cohen Apr. 16, 1957 2,802,127 Dobischek et al Aug. 6, 1957 2,842,706 Dobischek et al July 8, 1958 

